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Roundworms - Advanced Douglas Wilkin, Ph.D. Jennifer Blanchette Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2015 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: December 22, 2015 AUTHORS Douglas Wilkin, Ph.D. Jennifer Blanchette www.ck12.org C HAPTER Chapter 1. Roundworms - Advanced 1 Roundworms - Advanced • Describe the structural features of roundworms that distinguish them from flatworms. When most people picture a worm, do they picture a roundworm? Actually, they do not. Whereas flatworms are flat, roundworms obviously appear round. With over 80,000 species, there are plenty of different types of roundworms. But these are still not the earthworms most people picture when they think of worms. Characteristics of Roundworms (Nematoda) Roundworms make up the phylum Nematoda. This is one of the most abundant animal phyla, with greater than 80,000 known species (although not all of them have been classified). They range in size from less than one millimeter to 7 meters. Roundworms are found nearly everywhere, including aquatic and terrestrial environments as well as parasitically on or within a variety of plants and animals. In the next several sections we will review the anatomy of roundworms, highlighting the features specific to this phylum. Anatomy of Roundworms As the name implies, roundworms are cylindrical in shape. Like flatworms, roundworms have excretory, nervous, and reproductive systems. Another similarity to flatworms is that they lack an enclosed circulatory and respiratory system. The two most prominent features that distinguish roundworms from flatworms are the presence of a pseudocoelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an anus. A pseudocoelom differs from a true coelom in that the cavity is lined with mesoderm only on the ectodermal side. The presence of a body cavity allows circulatory fluid to flow freely throughout the body of the organism and facilitates the exchange of material between cells (including the release of waste). A complete digestive tract with separate openings for the mouth and anus allows the animals to simultaneously feed, digest, and eliminate waste. The main differences between flatworms and roundworms are summarized in the Comparison Table. 1 www.ck12.org TABLE 1.1: A Comparison of Flatworms and Roundworms Flatworms (Platyhelminthes) Flat body Primitive gut (1 opening) Acoelomate (no body cavity) Roundworms (Nematoda) Round body Complete digestive tract (2 openings) Pseudocoelem (partial body cavity) Roundworms have a thick substance called a cuticle on the surface of their bodies that is secreted by the outer epidermal cells. The cuticle is fairly rigid and limits the volume of the worm. This allows the build up of hydrostatic pressure from fluid accumulated in the worm and contributes to what is called a hydrostatic skeleton. The force of the hydrostatic pressure allows the worm to maintain its cylindrical shape. This hydrostatic force along with contractions from the muscles lining the pseudocoelom allows the worm to move along solid surfaces. As is the case for flatworms, there are both free-living and parasitic species of roundworms. The nervous system of nematodes, like that of flatworms, is simple and has a concentrated nerve center in the anterior, or head, region. Nematodes also have sensory organs connected to their nervous system. They generally have either one, termed the amphid, or two: an anterior amphid and a posterior phasmid. An example of a roundworm, the soybean cyst nematode, is shown in the Figure 1.1. FIGURE 1.1 Scanning electron micrograph (microscopic photograph) of a soybean cyst nematode magnified 1000x. Vocabulary • amphid: A circular depression situated laterally at the anterior end of nematodes and believed to be chemoreceptors. • coelom: A fluid filled cavity formed within the mesoderm; it forms between the digestive cavity and the body wall. • cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof covering over the aerial surfaces of a plant. 2 www.ck12.org Chapter 1. Roundworms - Advanced • hydrostatic skeleton: A structure consisting of a fluid-filled cavity surrounded by muscles; it is used to change an organism’s shape and produce movement. • phasmid: One of a pair of circular depressions situated laterally at the posterior end of nematodes and believed to be chemoreceptors. • pseudocoelom: A partially developed, fluid-filled body cavity that is lined with mesoderm only on the ectodermal side. Summary • The two most prominent features that distinguish roundworms from flatworms are the presence of a pseudocoelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an anus. • Roundworms have a thick substance called a cuticle on the surface of their body that is secreted by the outer epidermal cells. • There are both free-living and parasitic species of roundworms. Practice Use this resource to answer the questions that follow. • Introduction to the Nematoda at http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html . 1. 2. 3. 4. 5. 6. How prevalent are species of Nematoda? What groups are nematodes most closely related to? What is unique about a nematodes nerve and muscle cells? How are nutrients distributed within a nematode? What is cryptobiosis? Where can you find nematodes? Review 1. 2. 3. 4. 5. What are the main differences between roundworms and flatworms? What differentiates a pseudocoelom from a true coelom? Why is a body cavity so important as an evolutionary development? How is the roundworm’s digestive system superior to the digestive system of flatworms? What allows roundworms to maintain their shape and move? References 1. United States Department of Agriculture. http://commons.wikimedia.org/wiki/Image:Soybean_cyst_nematod e_and_egg_SEM.jpg . Public Domain 3